Field
Embodiments relate to the field of circuit protection devices, and more particularly to a metal oxide varistor for surge protection.
Discussion of Related Art
Over-voltage protection devices are used to protect electronic circuits and components from damage due to over-voltage fault conditions. These over-voltage protection devices may include metal oxide varistors (MOVs) that are connected between the circuits to be protected and a ground line. MOVs have a current-voltage characteristic that allows them to be used to protect such circuits against catastrophic voltage surges. Because varistor devices are so widely deployed to protect many different types of apparatus, there is a continuing need to improve properties of varistors.
An MOV device (the terms “MOV” and “varistor” are used interchangeably herein unless otherwise noted) is generally composed of a ceramic disc, often based upon ZnO, an electrical contact layer that acts as an electrode, such as a Ag (silver) electrode, and a first metal lead and second metal lead connected at a first surface and second surface, respectively, where the second surface opposes the first surface. The MOV device is also provided with an insulation coating that surrounds the ceramic disc and other materials in many cases. An example of an MOV found in the present market includes a ceramic disc that is coated with epoxy insulation, which has a high dielectric strength.
Notably, this type of MOV is typically restricted for operation at relatively low temperature, such as less than 85° C., and more particularly exhibits reliability problems when operated at bias humidity conditions such as 85° C., 85% relative humidity (RH) and high DC operating voltage. It is believed that the reliability problems experienced under such a bias humidity condition arise from the migration of silver electrode material used to contact surfaces of the ceramic body of the MOV, as well as from the interaction between the epoxy coating and ZnO ceramic. An example of the reliability problems is the increased leakage through the interface when an epoxy-coated MOV is operated at high temperature (at least 85° C.), high humidity conditions while applying DC operating voltage. Moreover, even under lower humidity conditions an epoxy-coated MOV may fail during operation at elevated temperatures, such as 125° C. It is with respect to these and other issues that the present improvements may be desirable.